1. Field of the Invention
This invention relates generally to a cryogenic tank and, more particularly, to a cryogenic tank that employs a heat sensitive cryo-valve, where the tank has particular application for storing liquid hydrogen in a fuel cell system.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is disassociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Many fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen in the air is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
In an automotive fuel cell application, the hydrogen is sometimes stored in a cryogenic tank on the vehicle, where the hydrogen is a liquid at very cold temperatures, such as 25° K. The cryogenic tank typically includes an inner tank and an outer tank having a vacuum and a multi-layer insulation (MLI) therebetween to limit heat penetration into the inner tank to maintain the liquid hydrogen in its supercold state. State of the art cryogenic hydrogen storage tanks on vehicles include a mechanical cryo-valve that selectively allows the hydrogen to be removed from the tank through a pipe. Cryo-valves are typically discreet valves in that they are entirely open or entirely closed. If no hydrogen is demanded, the valve is closed. Due to its size and construction, the control unit for the cryo-valve is positioned outside of the tank. Additionally, measures are taken to prevent ice forming condensation to occur on the valve. The cryo-valve typically includes a long valve stem to actuate the valve, which minimizes the heat path caused by the intrusion into the tank. Therefore, cryo-valves typically have a large mass and complex design. The cryo-valve could also operate as a pressure release valve for releasing pressure from the tank. However, a separate pressure release valve is typically provided for this purpose.
FIG. 1 is a plan view of a known cryogenic tank 10 including an outer tank 12 having a vacuum and an MLI and an inner tank 14, where the inner tank 14 stores liquid hydrogen at a low temperature. Additional devices (not shown), such as a pressure release valve and a filling line, would also be included with the tank 10. Hydrogen is emitted from the inner tank 14 through a pipe 16 to the fuel cell stack (not shown). The tank 10 includes a cryo-valve 18 that closes and opens the hydrogen path through the pipe 16 in response to a control signal. The cryo-valve 18 includes a valve stem 20 that actuates a valve plate (not shown) to open and close an opening in the pipe 16. A coil 22 controls the position of the valve stem 20. The length of the stem 20 minimizes the heat loss through the cryo-valve 18.
The inner tank 14 is heated by heat radiation and heat conduction to the environment through the outer tank 12. The heat radiation and heat conduction can be reduced by the combination of vacuum and the MLI, but cannot be completely prevented. Additionally, heat from the environment flows through the pipe 16 to the tank 14 and heats the liquid hydrogen therein. Also, heat from the environment gains access to the tank 14 through the cryo-valve 18.
Initially, the tank 14 is filled with liquid hydrogen having a temperature in equilibrium with the environmental pressure (1 bar). As the temperature of the hydrogen within the tank 14 increases, the pressure within the tank 14 increases. However, the pressure within the tank 14 is limited to a critical pressure level, which is the boil-off pressure. If the pressure within the tank 14 reaches the boil-off pressure, hydrogen must be released from the tank 14 in order to reduce the pressure. The cryo-valve 18 could also act as a pressure release valve for this purpose. The time from when the tank 14 is completely filled with hydrogen and the cryo-valve 18 is closed thereafter to when the boil-off pressure is reached in the tank 14 is the autonomy time. Because vehicles are sometimes not operated for extended periods of time, it is desirable to maximize the autonomy time by minimizing the heat losses from the tank 14.
It would be desirable to decrease the heat losses through the cryo-valve in a cryogenic tank, reduce the weight of the cryo-valve, and decrease the cost and complexity of the cryo-valve.